A magnetoresistive random access memory (MRAM) cell generally contains a non-magnetic conductor forming a lower electrical contact, a free magnetic layer, a tunnel barrier layer, a pinned magnetic layer and a second non-magnetic conductor. The free magnetic layer, tunnel barrier layer and pinned magnetic layer collectively form a magnetic tunnel junction (MTJ) device.
Directions of magnetic orientations in the magnetic layers of the MRAM cell cause resistance variations. Magnetic orientation in one magnetic layer is magnetically fixed or pinned, while the magnetic orientation of the other magnetic layer is variable so that the magnetic orientation is free to switch direction.
In response to the shifting state of the free magnetic layer, the MRAM cell exhibits one of two different resistances or potentials which are read by the memory circuit as either a “1” or a “0.” It is the creation and detection of these two distinct resistances or potentials that allows the memory circuit to read from and write information to an MRAM cell.
A bit of information may be written into the MTJ of an MRAM cell by applying orthogonal magnetic fields directed within the XY-plane of the MTJ. Depending on the strength of the magnetic fields, which are created by a current passing through the write line, the free magnetic layer's polarization may remain the same or switch direction. The free magnetic layer's polarization then may continue to be parallel to the pinned magnetic layer's polarization, or anti-parallel to the pinned magnetic layer's polarization.
The MTJ is in a state of low resistance if the overall orientation of magnetization in the free magnetic layer is parallel to the orientation of magnetization of the pinned magnetic layer. Conversely, the MTJ is in a state of high resistance if the overall orientation of magnetization in the free magnetic layer is anti-parallel to the orientation of magnetization in the pinned magnetic layer.
With reference to FIG. 1, a conventional MRAM cell structure 100 is depicted. The lowermost thin layer, or seed layer 102, is generally made up of tantalum (Ta). The next thin layer is ferromagnetic (FM) layer 104 which is the free layer. Free FM layer 104 is generally made up of a nickel iron (NiFe) alloy. As described above, it is free FM layer's 104 magnetic polarization that switches between being in a parallel state and an anti-parallel state with respect to the pinned layer(s) depending upon the strength of the magnetic field created by current passing through the write line of the memory circuit.
A tunnel barrier thin film layer 106 is shown on top of free FM layer 104. Tunnel barrier layer 106 is generally made up of aluminum oxide. A second pinned FM layer 108 is shown on top of barrier layer 106. Pinned FM layer 108 is typically formed of alloys of one or more of the following: Ni, Fe and cobalt (Co). A ruthenium (Ru) coupling layer 110 is formed on top of pinned FM layer 108 and couples the second pinned FM layer 108 with a third pinning FM layer 112. An anti-ferromagnetic layer 114 is then formed on top of the third pinning FM layer 112. Anti-ferromagnetic layer 114 is generally formed of iridium manganese (IrMn) or platinum manganese (PtMn).
In the MRAM cell 100 depicted in FIG. 1, the thickness of the second pinned FM layer 108 is represented by y and the thickness of the third pinning FM layer 112 is represented by x, where x and y can be equal or y could be thicker (where x and y are typically in the range of approximately 20–50 angstroms) for stability of the memory cell 100. The magnetization directions of the third pinning FM layer 112 and the second pinned FM layer 108 are anti-parallel. The relative thicknesses of the FM layers 108, 112 dictate which direction the magnetic material in the layers will be oriented. The third FM layer 112 is the pinned layer and the second FM layer 108 is the reference layer.
The anti-parallel status of the two FM layers 108, 112 is due to the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling of the reference layer 108. As depicted in FIG. 1, a portion of the demagnetization field from third FM layer 112 also passed through the free layer 104. The demagnetization field received from the third FM layer 112 creates an offset coupling effect at the free layer 104 that biases the orientation of the free layer in the same direction as the second pinned FM layer 108 and can affect the switching characteristics of the free layer 104 as well as increase the energy required to write a bit to the MRAM cell 100.
In addition, during fabrication of the MRAM cell 100 depicted in FIG. 1, the thin metal layers are typically formed by sputter deposition, evaporation or epitaxy techniques. When such methods are used, rather than being flat, the layers instead exhibit surface or interface waviness. This waviness of the surfaces and/or interfaces of the FM layers 104, 108, 112 is the cause of magnetic coupling between the free FM layer 104 and FM layer 108, which is known as topological coupling or Neel coupling. Just as the offset coupling described above, Neel coupling can affect the switching characteristics of the free layer 104 as well as increase the energy required to write a bit to the MRAM cell 100. Thus it is desirable to develop a MRAM cell having reduced offset coupling and reduced Neel coupling so as to alleviate the bias on the free layer.